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mx rider
Dodge Dakota
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5/11/2005
02:05:24
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 Subject: rearend gears and torque
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i have a 04qc 4.7 manual with 3.55 rearend how much more torque at the rear wheels would i get with the 3.90 or the4.10 gears
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.boB
Dodge Dakota
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5/11/2005
12:23:25
| RE: rearend gears and torque
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In theory, the 3.92 will get almost a 10% improvement. However, taller gears are less efficient, so it won't be quite that much.
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Kowalski
GenIII
5/11/2005
15:37:32
| RE: rearend gears and torque
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Taller gears are highway gears; the numerically larger gears are actually called lower gears. The rear gear is a torque multiplier - basically, 3.55 multiplies your motor'e torque by 3.55; 4.10s multiply it by 4.10, etc.
Lead, follow, or get out of the way
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.boB
Dodge Dakota
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5/11/2005
16:41:13
| RE: rearend gears and torque
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I wish it was that simple. If you do the math it's pretty clear it is not.
The 5.9 engine makes 345 ft/lb at the crank shaft (on a good day). You lose 15% through the drive train, leaving you with about 290 ft/lb. Multiply that x 4.10, and you get almost 1190 ft/lb to the pavement. At 5,000 rpm you would be making about 1,000hp.
That would be nice, but I don't think my truck is making anywhere near that.
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yes
Dodge Dakota
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5/11/2005
16:56:45
| RE: rearend gears and torque
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yes, but don't forget that the box of gears between the engine and the rear end.
It's called a transmission.
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.boB
Dodge Dakota
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5/11/2005
17:06:50
| RE: rearend gears and torque
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Right, that would multiply the torque even more in 1st and 2nd. The math gets even worse! And don't forget the torque converter, at low speeds it also increases torqe. Oh my god! I have a 2500hp Dakota!
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torque
Dodge Dakota
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5/11/2005
17:23:23
| RE: rearend gears and torque
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.boB
Gears are torque (not horsepower) multipliers. The torque with any ratio is rated at the axle, Engine torque minus drive train loss times gear ratio. The effective "pulling power" is also affected by the loaded rear tire radius. No flame, just facts.
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Kowalski
GenIII
5/11/2005
17:57:40
| RE: rearend gears and torque
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Yup. See .boB - it really is that simple, its your math that's flawed. Gearing is a similar principle to leverage, that's why with a long enough lever you can move something that's heavier than you are. The object will move a little while you move the lever a lot; but that doesn't make you heavier than the object !
Lead, follow, or get out of the way
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.boB
Dodge Dakota
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5/11/2005
22:01:55
| RE: rearend gears and torque
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You are absolutly correct - I'm not a math whiz. And I agree with you that a higher gear ratio is equal to a longer torque arm. That makes perfect sense; simple high school physics. What I don't understand, is how you transfer power down through the drive train with repeated torque multiplication steps, and actually lose power. I've never reasearched it to figure out why, I guess because I just don't care enough to dig in to it.
But the point is, a 3.55 rear gear does not simply multiply your engines torque 3.55 times. That just doesn't work. 1 minute with a calculater will tell you that. I don't know the why of it, I just know that's true. Even if you use the simplest form of the equation and eliminate all parasitic losses, it still doesn't work.
As for HP, it is not measured it's calculated. (torque x RPM)/5252=HP That's why every hp/torque curve you see crosses at 5252 rpm's. It was originally developed as a way to sell machinery to farmers, by comparing the power the new fangled gas engine makes compared to the standard work horse.
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torque
Dodge Dakota
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5/11/2005
22:51:28
| RE: rearend gears and torque
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.boB quote "What I don't understand, is how you transfer power down through the drive train with repeated torque multiplication steps, and actually lose power. I've never reasearched it to figure out why, I guess because I just don't care enough to dig in to it."
I don't completely understand it either. I know that at given rpm's and temperature the transmission / transfer case losses are more/less. I just know that as Kawolski indicated, torque is the basic. Our trucks produce torque at moderately low rpm's for towing. They're designed to "pull the load". With a proper camshaft change, you can adjust the "torque bandwith" to there by controll shift points. You want to shift as the torque starts to fall off and have the proper gearing to "connect" within the torque band through the next gear.....
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Bob Lincoln
Dodge Dakota
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5/12/2005
08:25:22
| RE: rearend gears and torque
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"As for HP, it is not measured it's calculated. (torque x RPM)/5252=HP That's why every hp/torque curve you see crosses at 5252 rpm's. It was originally developed as a way to sell machinery to farmers, by comparing the power the new fangled gas engine makes compared to the standard work horse."
WHAT? What have you been smoking? First of all, HP and torque curves don't all cross at 5252 RPMs, that's BS. Second, your "formula" makes no sense at all and fails with every set of published numbers I put in. Reason is, ft-lb/second equals power, your units are all wrong.
"The horsepower unit (hp) was originally defined by James Watt (1736-1819) as exactly equal to 550 ft-lbf per second". No gas engines then. You fail history as well as physics.
And HP and torque published numbers are measured at the rear wheels since 1971, not at the crank.
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Dan M
Dodge Dakota
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5/12/2005
08:49:11
| RE: rearend gears and torque
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Here's a bit further explanation of what Bob Lincoln was saying.
http://www.v8914.com/Horsepower-v-torque.htm
- Dan M
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.boB
Dodge Dakota
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5/12/2005
09:27:53
| RE: rearend gears and torque
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"WHAT? What have you been smoking? First of all, HP and torque curves don't all cross at 5252 RPMs, that's BS. Second, your "formula" makes no sense at all and fails with every set of published numbers I put in. Reason is, ft-lb/second equals power, your units are all wrong."
Havn't done your research, have you?
Here's the point. If gears simply multiply torque, why do we lose so much power through the drivetrain? If you do the simple math, it just doesn't fit. Take a 300 ft/lb engine, in 4th gear (1:1), and a 4.10 rear gear. The simple math would tell you that you're applying 1230 ft/lb at the rear wheels. When have you ever seen that on a chassis dyno?
Do the math backwards. A 360 Dakota is advertised as making 345 ft/lb. That's measured at the rear wheels. If you have a 3.55 rear gear, that means the engine is making 97 ft/lb at the crank shaft. Does that make sense to anyone?
The point is that it's much more complicated than simply multiplying torque x gear ratio.
Dan, thanx for that link. It was interesting reading. When technology finally reaches the point that a CVT will actually work and survive, everyone will jump on the bandwagon.
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hmmm
Dodge Dakota
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5/12/2005
14:40:18
| RE: rearend gears and torque
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"When technology finally reaches the point that a CVT will actually work and survive, everyone will jump on the bandwagon."
CVT's have been perfected and last just as long as any transmission going. The problem is on of marketing and paradigm shift. Also, cars with CVT's tend to be more upscale and manual shifters more low brow. i.e. uneducated and lower income earners.
.boB, I think the concept you are having trouble wrapping your head around is the notion that torque through the power train remains constant, minus a few % for losses but what varies is RPM.
(T=FD)
Given the above formula what is varying is the distance over which the work is being done. Gears do not change net torque. They do not multiply torque. That's an over simplification. Gears vary speed. If you do the calculation properly you'll see the relationship between speed (RPM), power and torque which all relates to a force moving over a distance.
And contrary to what Mr. Lincoln says, torque and horsepower curves always cross at 5252RPM, if the power is being measured in horsepower and the torque in foot pounds and both lines are shown on the same axes. If they don't cross at 5252RPM then they have been calculated incorrectly. Horsepower = Torque x rpm / 5252.
Just because you can't understand something, that doesn't makes those who do understand wrong. Just frustrated enough to kill.
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.boB
Dodge Dakota
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5/12/2005
15:48:07
| RE: rearend gears and torque
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Hmmm, thanx for that. I clearly don't have a solid grasp of all the intricacies of how this works. All my knowledge comes from automotive experiance. And that's why the simple math doesn't make sense. It's just too simple. I knew there had to be a lot more to it.
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GraphiteDak
GenIII
5/13/2005
00:49:33
| RE: rearend gears and torque
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Screw all of that nonsense!!!!
When I swap in my 4.10's later I'll tell you what I measure on my BUTT DYNO!!! 
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furball69
Dodge Dakota
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5/13/2005
01:54:13
| RE: rearend gears and torque
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.boB
Its known as frictional losses. The engines power is at it's peak at the flywheel, which then goes through a transmission, driveshaft, differential, and two axles before the power is put to the wheels, not to mention each wheel weighs what, 40 - 50 pounds.
Frictional losses are typically around 15 - 20%
Bob Lincoln,
manufacturers don't measure RWHP, they measure NET HP at the crank WITH accessories, since 1972, before that it was Gross HP at the crank with no accessories.
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furball69
Dodge Dakota
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5/13/2005
05:31:53
| RE: rearend gears and torque
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To make it maybe a little easier to understand, imagine this:
put the truck up on blocks, take the engine out and hook bicycle sprocket to the torque converter, now hook up the same size sprocket to a stationary bike with a chain between the two, put the tranny in gear and start peddaling. If there is zero friction and loss the pedalling will be effortless; I bet it isn't.
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ptschett
Dodge Dakota
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5/13/2005
18:40:12
| RE: rearend gears and torque
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In college I was part of a team building 1/4 scale tractors for a national competition held by the ASAE. We built 2 and 4 wheel drive tractors (depending on that year's ruleset) using 26" tall drive tires and a 16 HP (rated @ 3600 RPM) Briggs & Stratton V-twin. The 4wd (actually a 6 wheeler with 4 rear drivers) weighed about 1000 lbs and could exert more than that in tractive effort given proper ballasting. We never did find out what our peak output force would be but I bet it could have got 2000 lbs. If that little tractor had a chance of getting 2000 lb-ft at the axle, why not a pickup with 15 times the HP and torque and geared very roughly that much faster?
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furball69
Dodge Dakota
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5/13/2005
19:21:58
| RE: rearend gears and torque
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The definition of torque is: a force which causes rotation.
The formula for calculating torque of a motor is: T = (5252 * HP) / RPM
Lower gears (higher numerically) means higher RPM; as you can see form the formula, you divide by the engines rpm, so the higher the rpm the lower the torque.
Alternatively, you can maintain the same rpm with the lower gears and move the vehicle more slowly. which means less horsepower. Horsepower is a measurement of moving a certain weight, a certain distance over a certain period of time.
I'm reasonably sure there may be something wrong with my logic, I'm sure someone will correct me if there is.
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ah
Dodge Dakota
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9/20/2005
11:39:51
|  RE: rearend gears and torque
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i hate to tell you but your 16hp tractor didnt make 2000 lbs of torque thats more then a semi truck makes. maybe you should keep the tractor in the field and keep growing me some corn! cause u aint to bright. i means brains wise.
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